This invention relates to a system for detecting or determining, in a receiver of a radio communication system or link a frequency and/or phase difference generally produced between each pair of communication signals reaching the receiver in a common frequency band through at least one repeater station of the link and for thereby enabling cancellation or compensation of interference which inevitably occurs between the communication signals before reaching the receiver.
In a conventional terrestrial microwave communication link comprising a transmitting or origination station, at least one repeater station, and a receiving or destination station, the frequency and/or phase difference has given rise to no serious problems in general because the communication signals reaching the receiver through different propagation paths or communication channels have been well separable by directivities of antenna beams and/or carrier frequencies of the communication signals. In order to more effectively use the frequencies, it has become the practice to resort to a narrower carrier frequency spacing in which the communication signals are not readily separable by the carrier frequencies. Even for these communication signals, separation is possible by the antenna directivity discrimination and also by polarization discrimination which makes use of carrier electromagnetic waves or signals of cross-polarized modes. The separation is, however, likely to deteriorate the quality of the link performance when interference becomes serious between the communication channels as a result of various inevitable adverse effects caused by the conditions of the communication channels. For example, rain inevitably introduces differences in the phase and the amplitudes of the cross-polarized carrier signals and consequently in the mode of polarization of the communication signals to damage the polarization discrimination. Variations inevitable in the atmospheric conditions result, in turn, in variations in scatter, reflection, and other manners of propagation of the carrier signals to degrade the directivity discrimination and also the polarization discrimination. The situations are similar even in a satellite communication link in which transponders carried by a satellite serve as repeater station. It is therefore necessary to detect the interference in a receiver of the destination and/or repeater station to compensate the interference in the destination, repeater, and/or origination station.
A system for detecting interference signals resulting from the inevitable interference between cross-polarized electromagnetic waves, such as a dextrorotary and a levorotary elliptically polarized waves, of a common frequency band is disclosed in U.S. Pat. No. 3,735,266 issued to Noach Amitay, an assignor to Bell Telephone Laboratories, incorporated, or in U.S. Pat. No. 4,090,137 issued to Shoji Soma and Ikuro Sato, assignors to the present assignee. Pilot carrier signals of different frequencies are transmitted as the respective cross-polarized waves. In a receiving station, the pilot carrier signals and interference signals occurring across the cross-polarized channels are detected for comparison between their respective phases or amplitudes to determine levels of the interference signals. The disclosed system is applicable in general to a communication link comprising a transmitting station and the receiving station and no repeater station although possibility of inclusion of one or more repeater stations is mentioned in the referenced United States Patent. For example, the proposed system is applicable in a satellite communication link only to a down link and not to a whole link comprising an up link and the down link.
The incapability of detecting the interference signals in a destination station of a radio communication link comprising a repeater station results from the fact that the repeater station usually carries out frequency conversion of the communication signals arriving thereat generally with another common frequency to give rise to the frequency and/or phase difference mentioned hereinabove. This is typically the case when the communication signals are received at the destination station through a plurality of repeaters. Detection and compensation of the frequency and/or phase difference are accordingly mandatory.